Numerous types of analog-to-digital (A/D) conversion products are presently available. Many of these products are sold as bare circuit boards which allow incorporation of various different A/D chips and are used with a variety of data gathering platforms. Other products are sold as complete systems that can be connected to a personal computer (PC) and which then allow the user to collect data from a variety of sources. However, typical prior art products are deficient in their capability for low noise or substantially noise free operation, which inhibits their ability to adequately function in data acquisition systems for monitoring small DC voltage signals.
Some modern data acquisition systems interface directly with a PC. This allows for the efficient display and processing of the results, often in real time (or just a few milliseconds delay from true real time). The PC also provides a convenient way to store the large volumes of data, obtained from measurements, for later post-processing analysis.
Data acquisition systems typically combine input circuitry with an analog-to-digital converter (ADC) chip. Generally, the input circuitry of such systems receive input signals from a variety of transducers and the ADC produces digital outputs proportional to the varying level of the voltage of the input signals. ADCs typically have a full scale input voltage range of .+-.2.5 to .+-.5 V. To process a small DC voltage signal (i.e., a signal significantly less than the full scale input of the ADC), traditional systems typically employ one or more signal amplifiers to increase the small input voltage signal to correspondingly match the full scale voltage input range of the ADC chip. One drawback to this approach is that the amplification process introduces additional errors (i.e., noise) into the analog signal being amplified. Another common source of error and noise is the use of a multiplexer to sample several different analog inputs with a single ADC chip. This causes noise and spurious signal offsets due to the capacitive charge injection caused by switching between different wires containing the different analog inputs (e.g., often this noise plus offset is on the order of 10 mV peak to peak; see also FIG. 10) and can prevent the accurate measurement of small signals.
If the amplification is done correctly, then the full scale output of the amplified signal will match the full scale input in the A/D converter or at least use a significant portion of the ADC's full scale input voltage range. Therefore, amplification enables a user to view small signals with high resolution, since "extra bits" are added to the resolution. Thus, amplification of a .+-.0.5 V signal to a .+-.5.0 V signal which is converted in a .+-.5.0 V full scale ADC provides a 10 time increase in resolution.
Although the small input signals are typically filtered prior to amplification in an attempt to reduce the amount of noise that will be amplified, these errors cannot be totally eliminated; and they are amplified along with the signal. Moreover, the amplification process itself adds noise to the signal. Therefore, as the signal is amplified to increase the resolution during a conversion process, more noise is added to the signal. This tends to make it more difficult to accurately process small signals and adds uncertainty to the resulting digital output.
A typical "error-budget" for a small transducer signal (or measurement signal), which is amplified to match the full scale input range of an A/D converter chip ("Signal.sub.atA/D ") in a general PC-based A/D converter system, is a follows:
1. the transducer's signal or real measurement ("Signal.sub.Device "); PA1 2. the transducer's inherent, self-generated noise ("Noise.sub.Device "); PA1 3. environmental noise which radiates into the shielded and unshielded wire leads that connect the transducer to the amplifier ("Noise.sub.AmpLeads "); PA1 4. environmental noise which radiates into the shielded and unshielded wire leads that connect the amplifier to the A/D input ("Noise.sub.A/DLeads "); PA1 5. noise from the internal components that make up the amplifier ("Noise.sub.AmpInt "); PA1 6. noise from the external components that make up the amplifier ("Noise.sub.AmpExt "); PA1 7. span errors from temperature induced drift that affect the amplifier's internal gain stage ("Gain.sub.AmpInt "); PA1 8. span errors from temperature induced drift that affect the amplifier's gain through the external components ("Gain.sub.AmpExt "); PA1 9. input offsets in the amplifier which are multiplied by the gain ("Offset.sub.AmpInput "); PA1 10. output offsets in the amplifier which are constants ("Offset.sub.AmpOutput "); and PA1 11. spurious signal errors due to multiplexing several analog inputs which have differing line capacitances and other sources of errors ("Signal.sub.Spurious "). PA1 Number of Bit-counts from the A/D as a Digital Output=.+-.Integer[(Signal.sub.atA/D /V.sub.ref).times.2.sup.n ] PA1 Where n=the number of bits of resolution.
This error budget yields the following equation for the signal received at the input to the A/D chip: ##EQU1## where the Gain.sub.Ideal equals the amplifier's desired gain multiplier. Depending upon the actual circuit being evaluated, the Signal.sub.Spurious term may or may not be multiplied by the gain of the three Gain terms.
This Signal.sub.atA/D leads to the following equation for the resulting output resolution for the signal:
The above-described error budget is not the only source of errors. Additional measurement errors in the output signal are caused by the A/D convertor's voltage reference circuitry. Generally, A/D circuits use a voltage reference ("V.sub.ref ") signal to set the full-scale input of the ADC chip. The ADCs use a variety of comparison techniques to arrive at the number of bit-counts a signal value represents when compared to the full scale represented by the value of V.sub.ref. The V.sub.ref is assumed to represent the maximum number of bits in the ADC. For example, if the voltage reference is 100 and the Signal.sub.atA/D is 95; then the bit count in an 18 bit ADC is 249,036; on the other hand, if the Voltage reference is actually 101, then the bit count should be 246,571--an error of 2,465. Therefore, small errors in the V.sub.ref result in errors in the digital output signal.
The affects of all these errors on the conversion of a small signal from analog to digital can be demonstrated through some simple tests. One such test measures the noise present in an A/D circuit when there is a zero voltage level input applied to the ADC. This is called a "zero voltage level noise test." To perform the test for an actual real world application, the leads connecting the sensor to the PC based A/D circuit are disconnected and the positive and negative leads are tied together so that there is no voltage differential between them. Specifically, the leads are tied together as close to the sensor and as far from the PC as possible. This is because most sensors are located a distance from the PC in real world applications to prevent noise from the PC containing the A/D circuit from radiating into the sensor setup. Often the lead length distances are on the order of 6 to 20 feet or more. Typically, shielded cable is used with the shield tied to analog ground. Theoretically the results of this test should yield zero volts because any noise which radiates into the shielded cable or any capacitive effects on the cables in this zero voltage level noise test would be canceled out via the differential inputs, since the positive and negative sides of the ADC input see the same input voltages induced by the noise.
Most manufacturers perform a different version of this test, and the results of this alternate version of the test are often far better than the results achievable in the real world test described above. In the alternate version of this test, the manufacturer shorts the input pins of the ADC chip together as close as possible to the ADC chip, not as close as possible to the sensor or area of actual usage. Usually, the lead length is only a few inches long or even less than one inch for this test. This short lead length is usually comprised of the metallic traces on the actual printed circuit board to which the ADC chip is mounted and terminates at the connector mounted on the same printed circuit board. In the event that a multiplexer is present to sample several analog inputs to a single ADC chip, the short lead length also substantially reduces the effects of the line capacitance. Since the input signal is essentially 0 volts, the charge injection problem is minimized and a very good sounding specification may placed on the equipment. Therefore, no user can directly connect a device to the ADC circuit boards under the same conditions, and a circuit specification is determined for the system that has no bearing in the real world.
A typical real world input voltage test result is shown in FIG. 10. The results were generated by a conventional Analogic Corp. (Peabody, Mass.) High Speed DAS 16-bit analog-to-digital data collection board (Part No. HSDAS-16). The board is specified as having a resolution of 38 .mu.V (or 38.times.10.sup.-6 V) and is purported to have the capability to take accurate readings below 1 mV (or 1000.times.10.sup.-6 V). The board's specification also purports to have a maximum noise level of 76 .mu.V. However, in the test represented by FIG. 10, the product exhibited an inherent noise and offset of about 250-1000 .mu.V RMS, and the peak-to-peak noise level often exceeded 10 mV. The larger peaks were the result of a multiplexer charge injection as different channels were scanned. During the test, the nominal on-board amplifier gain was set to 4.0 and approximately 6-8 feet of cable length was present between the area where the leads were tied together and the ADC chip on the printed circuit board. This long lead length was necessary because the PC radiates environmental noise that could be detected by the transducer to be used in this particular test setup. Also, this test setup involved the use of fluids and it was desirable to have the PC as far away as possible from the source of a potential spill or leak. The readings on eight differential inputs were scanned during this test.
In traditional ADC circuits, such as the HSDAS-16, the multiplexer is used to alternately measure several different analog voltage sources using a single ADC chip. The multiplexer also helps to protect the ADC chip from high voltages, such as a static discharge, that might damage the ADC chip.
Low pass filtering can help to smooth out the noise, but it does not have a significant affect on the offsets or uncertainty in the readings. The plot in FIG. 10 shows the results in only one channel, but each tested channel had a similar output curve with the only difference being the amount of the offset voltage errors.
Another test which can be performed is called a "fixed response transducer test". In this test, a pressure transducer is surrounded by a shielded, grounded metal case which is connected to the A/D data acquisition system. The pressure transducer is then given a thermally stable fixed head height of input pressure from a static column of water at room temperature. The thermal mass of the fluid is sufficient to prevent any measurable thermal drift in the sensor over a 1 to 10 minute test period. The fluid temperature during the test is monitored with a thermistor to within 0.1.degree. C. The output of the transducer is then measured for noise. It is assumed that offsets and span errors cannot be measured with certainty, but they will still be within the transducer data. Tests performed on the Analogic system described above, yielded results which are quite similar to those shown in FIG. 10, but at a baseline voltage closer to the full scale analog signal from the pressure transducer (i.e., about 100 mV).